Magnetospheres

This chapter concludes with a short account of a particular consequence of a planetary body having a substantial magnetic field. The magnetic field of the planet then interacts with the solar wind to create what is called a magnetosphere around the planet. Very complex mechanisms are involved in magnetospheres, and so the approach here will be qualitative and brief.

5.4.1 An Idealised Magnetosphere

Figure 5.11 shows a comparatively simple magnetosphere that will introduce the essential features. To the left, the solar wind is flowing in interplanetary space, and it is undisturbed by the planet. The magnetic field in the wind in this particular case is perpendicular to the flow, as also, for simplicity, is the magnetic axis of the planetary field. If the field in the wind were static then the interplanetary field would simply be the sum of the undisturbed wind field and the undisturbed planetary field. But the wind field is entrained in the wind - this is because the wind is a plasma, i.e. it is sufficiently ionised for copious electric currents to flow in it. Entrainment means that the wind carries the magnetic field along with it. As a result, the interaction of the wind with the planetary magnetic field gives a different outcome which we shall now explore.

The solar wind 'sweeps' interplanetary space clean of the planetary field except in the vicinity of the planet. On the upwind side of the planet (to the left in Figure 5.11) there is a roughly hemispherical boundary outside of which there is only the interplanetary field. On the downwind side the boundary stretches out into a long magnetotail. Within the boundary the planetary field near the planet is as if there were no wind field, but the planetary field gets more and more distorted as the boundary is approached. The boundary is called the magnetopause, and the volume it encloses is called the magnetosphere. The 'sphere' part of the name is to be interpreted as the planet's magnetic sphere of influence, rather than as a description of the shape of the boundary.

There are three sorts of magnetic field lines: there are those that start and end on the planetary surface, distorted though they may be; there are those that never encounter the planet; and there are those that start on the planet but, in effect, connect with the wind field and therefore never return to the planet - these are called reconnected lines.

The size of the magnetosphere is characterised as the distance from the centre of the planet to the upwind magnetopause. This distance is proportional to ^,1/3/(n1/6v1/3), where ^ is the magnitude of the magnetic dipole moment of the planet, v is the wind speed, and n is the number density (number per unit volume) of the charged particles in the solar wind (mainly electrons and protons). Though v does not vary much with the heliocentric distance, n diminishes as this

distance increases. Also, n varies with solar activity, and so the size of the magnetosphere also varies, being a maximum when n is a minimum.

Because the solar wind approaches the magnetopause at the high speeds around several hundred kilometres a second, it gets a rude shock at a boundary outside the magnetopause, called, appropriately, the bow shock. (The 'bow' is by analogy with a related phenomenon created on the surface of water as the bow of a boat moves through the surface at speeds greater than the speed of the surface ripples.) Between the bow shock and the magnetopause the solar wind is rapidly decelerated, the flow becomes turbulent and the wind plasma is strongly heated. The wind flows around the magnetopause, with very little of the plasma entering the magnetosphere. The region between the bow shock and the magnetopause is called the magnetosheath.

The small fraction of the solar wind plasma that enters the magnetosphere is only one source of magnetospheric plasma. Another is cosmic rays (Section 3.3.3). These highly energetic charged atomic particles readily cross the magnetopause, and though most of them pass out again, a small proportion is trapped. Furthermore, a small fraction of the cosmic rays collides with the atmosphere of the planet, or with its surface if it has no atmosphere. This results in the ejection of particles, and these include neutrons that decay into protons and electrons, many of which are then trapped in the magnetosphere. Yet another source of plasma is a slow leak of particles from the planet's upper atmosphere, both in the form of plasma and in the form of neutral atoms that subsequently become ionised.

Magnetospheric plasma is not uniformly distributed, but becomes concentrated towards the plane of the magnetic equator, where it constitutes the plasma sheet (Figure 5.11). Belts and toruses of plasma surrounding a planet can also occur.

Though the magnetospheric plasma is being added to all the time, there are also losses, outwards to interplanetary space, and inwards to the planet. Among the latter are energetic ions and electrons that reach the upper atmosphere and excite atoms there. The resulting emission of optical radiation is called an aurora. Aurorae are concentrated in a ring around each magnetic pole. Large fluxes of energetic electrons plunging into the upper atmosphere generate radio waves with wavelengths of the order of 10-100 metres. Such decametric radiation emanates from the Earth and from the giant planets, and as early as 1955 indicated that Jupiter has a powerful magnetic field. Aurorae and decametric radiation are intermittent phenomena, depending on the strength of the solar wind. Other radio waves, with wavelengths of the order of 0.1-1 metre, are generated in the magnetosphere by electrons travelling at high speeds. This is called synchrotron emission.

5.4.2 Real Magnetospheres

In the Solar System, the magnetic dipole moments of the Earth and of the giant planets are far larger than those of any other planetary body (Table 4.2), and correspondingly they have the most extensive magnetospheres. Their magnetic axes are not perpendicular to the solar wind flow, nor on the whole is the solar wind's magnetic field. Nevertheless, the general form of the magnetosphere in each case is roughly as in Figure 5.11, and there is also a plasma sheet and plasma belts.

Figure 5.12 shows the typical form of the magnetosphere of the Earth. There are two main plasma belts around the Earth - the Van Allen radiation belts, named after the American physicist James Alfred Van Allen (1914-2006) who discovered them in 1958. The inner belt consists largely of protons and electrons. These come from the solar wind, and also from the Earth's upper atmosphere partly through the action of cosmic rays. The outer belt is more

Bow shock

Magnetopause

Bow shock

105 km

Figure 5.12 The Earth's magnetosphere.

Inner Van Allen radiation belt

Outer Van Allen radiation belt

Plasma sheet

105 km

Figure 5.12 The Earth's magnetosphere.

tenuous, and the particles are less energetic. It is populated largely by the solar wind. Within the inner belt there is a third belt in which cosmic rays are prevalent.

There is also a plasma sheet (Figure 5.12). This has a low density, and is hot, the temperatures being 1-5 x 107K. It is fed largely by the solar wind.

□ What are the main constituents of the plasma sheet?

Being a sample of the solar wind, its main constituents are electrons and protons. Energetic electrons from this sheet find their way by various means into the upper atmosphere, particularly in a ring around the magnetic poles, where reconnected magnetic field lines intersect the ionosphere (Section 9.2.2). There the electrons can give rise to decametric radiation, and (along with other charged particles) also make a significant contribution to aurorae - the aurora borealis (Plate 26) in the northern hemisphere, and the aurora australis in the southern hemisphere. When the solar wind is strong, as at times of high solar activity, the ring widens and aurorae are then seen further from the Earth's magnetic poles, down to about 70° or so in magnetic latitude. The magnetic axis is tilted by about 11.5° with respect to the rotation axis (Figure 4.6), and so the corresponding geographical latitude depends on longitude. The auroral displays are at an altitude of only about 80-300 km, and so the tropics are not the place to go to see aurorae! Figure 5.13 shows Jupiter's magnetosphere.

□ Why is Jupiter's magnetosphere bigger than that of the Earth?

It is bigger because Jupiter is further from the Sun, and so the number density of charged particles in the solar wind is smaller (see the expression in Section 5.4.1), and because the magnetic dipole moment of Jupiter is 20 000 times larger than that of the Earth (Table 4.2). When the solar wind is particularly weak the upwind magnetopause can be about 100 Jovian radii from Jupiter. If we could see such a magnetosphere from Earth with Jupiter at opposition the magnetopause would be appear like a disc with an angular diameter about 2.6 times that of the full Moon. The magnetotail can extend beyond the orbit of Saturn.

The Jovian magnetosphere is particularly rich in plasma - in the denser regions the human body would quickly receive a lethal dose of ions. This richness is a result of copious internal sources, notably the volcanoes of Io, but also the Jovian upper atmosphere and the surfaces of Jupiter's satellites and ring particles. Ions and electrons are ejected from the satellites and rings by cosmic rays. As well as being sources of plasma, the satellites and rings also remove plasma

Bow shock

Magnetopause

Magnetic axis

Plasma belt

Magnetic axis

Io plasma torus

Solar

Solar

Plasma belt

Plasma sheet s

3 x 106 km

Figure 5.13 The Jovian magnetosphere.

particles that collide with them. The solar wind is not an important source of the magnetospheric plasma, except near the magnetopause and far out in the magnetotail. It can then be shown that the plasma sheet must be a result of leakage from the plasma belt. Leakage occurs preferentially at the magnetic equator, where the magnetic containment is weakest. This domination of internal sources of plasma is a result of electric fields in the magnetosphere plus the rapid rotation of Jupiter - the details are beyond our scope. Jupiter also displays aurorae, with the same basic cause as for the Earth.

Beyond Jupiter there are three more planets with extensive magnetospheres: Saturn, Uranus, and Neptune. The magnetosphere of Saturn is intermediate between that of the Earth and Jupiter in extent and plasma content. Sources and sinks of plasma include the rings and the satellites. Saturn also displays aurorae. Compared with the Voyager encounters in 1980 and 1981, Cassini in 2005 found no detectable changes in Saturn's internal magnetic field. The magnetosphere had changed in extent somewhat, but in line with the variable solar wind.

The considerable magnetospheres of Uranus and Neptune have some peculiarities arising from the large angles between their magnetic and rotation axes (Figure 4.6), and in the case of Uranus from its large axial inclination, but in terms of the above discussion no new major phenomena are encountered.

Question 5.9

Outline the consequences for the Earth's magnetosphere

(a) if the Earth had no atmosphere;

(b) if the speed and number density of charged particles in the solar wind were reduced. 5.5 Summary of Chapter 5

The terrestrial bodies are dominated by silicates, and by iron-rich compounds, including iron itself (see Figures 5.1, 5.5, and Io and Europa in 5.7). They are all differentiated, with iron-rich cores and silicate mantles. Europa has a thin icy crust, mainly water ice, underlain by an ocean of salty liquid water or slush, whereas the other terrestrial bodies have rocky crusts. The Earth's interior is by far the best known, and it is clear that there is an outer core mainly of iron that is hot enough to be liquid. This outer core is the source of the Earth's powerful magnetic field. All but the uppermost part of the Earth's mantle is an asthenosphere, and it is probably undergoing solid state convection. It is thought that the other terrestrial bodies also have warm interiors (Tables 5.2 and 5.3).

Internal temperatures in all of the terrestrial bodies are raised above the values they would have in equilibrium with solar radiation. In all cases, except Io and Europa, this is almost entirely through the effects of primordial energy sources and heat from long-lived radioactive isotopes. The temperatures decrease as the size of the body decreases, with the exception of Io, which, in spite of its small size, has an interior hot enough for silicate volcanism. This is because Io has a dominant tidal component in its internal energy sources. Europa is less tidally heated, being further from Jupiter, but this makes a contribution crucial to sustaining a salty ocean/slush.

Pluto, and the remaining large satellites (Figure 5.7), differ from the terrestrial bodies in having a much larger proportion of icy materials - they are icy-rocky bodies. They are thought to be differentiated into icy-rich mantles, and rocky-rich cores. Ganymede, Callisto, Titan, and perhaps Triton might well be liquid over some depth range. Eris is a bit larger than Pluto, and is icy-rocky, but as yet we know nothing of its interior.

The remaining satellites have Titania as their largest member. Most of them consist of roughly equal masses of icy and rocky materials. The very smallest satellites are unlikely to be differentiated.

The four giant planets (Figure 5.9) are dominated by hydrogen, helium, and icy materials, the proportion of hydrogen and helium being considerably greater in Jupiter and Saturn than in Uranus and Neptune. For all four bodies the hydrogen-helium ratio for the whole body is thought to be similar to that in the young Sun.

Jupiter and Saturn are each differentiated into an icy-rocky core, a mantle of metallic hydrogen, and an envelope of molecular hydrogen (H2), with helium (He) as the next most abundant component in the mantle and envelope. The boundaries between mantle and envelope are fuzzy. Saturn has a substantial icy-rocky core but in Jupiter the core might be very small, even absent, the heavy elements then being more uniformly distributed.

Uranus and Neptune have deep atmospheres, like Jupiter and Saturn, dominated by H2, with He the next most abundant component, but with more icy materials. These overlie predominantly icy cores. Though models of Uranus and Neptune are poorly constrained, we can be sure that because of the lower pressures there is no metallic hydrogen.

The interiors of all four giants are hot (Table 5.4), and they all have very large magnetic dipole moments, originating in the metallic hydrogen mantles in Jupiter and Saturn, and in icy mantles in Uranus and Neptune. The IR excess of Jupiter is largely accounted for by primordial accretion and early differentiation. A substantial supplement of ongoing differentiation is required to account for the IR excesses of Saturn and Neptune. In Saturn the differentiation is the separation of helium from hydrogen in the metallic phase. This is happening faster in Saturn than in Jupiter because of its lower internal temperatures. In Neptune the nature of the differentiation is unclear. Uranus has a very small IR excess, and if indeed its interior is hot, then the outward energy transfer rate is somehow being reduced, perhaps through the suppression of convection over some range of depths by a vertical composition gradient.

A planetary body with a substantial magnetic field will interact with the solar wind to produce a magnetosphere. Beyond the magnetosphere the solar wind sweeps space clean of the planetary field. Within the magnetosphere the planetary field becomes increasingly dominant as the planet is approached. There will be a variety of plasma in a magnetosphere, much of it concentrated into a plasma sheet and plasma belts or toruses. Interaction of the plasma with the upper atmosphere can produce aurorae.

6 Surfaces of Planets and Satellites: Methods and Processes

When we wonder about other worlds, it is usually their surfaces to which our thoughts first turn. After all, it is the surface of one particular world upon which we live, and there is a fascination with exploring terrestrial landscapes that differ from those of our own region. Even greater is the immediate fascination with the landscapes of other worlds. Such individual landscapes will be explored in the two chapters that follow this one. In this chapter we shall outline some of the methods of investigating surfaces, and the various processes that have made the surfaces as they are. The giant planets do not have surfaces in the generally accepted sense, so they are excluded from these chapters.

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